9 SPRINKLERS

9.1 Industrial Buildings Excluding High Racked Storage
Warehouses
Whilst it is generally agreed that the operation of
sprinklers will reduce the fire size and the amount
of heat released, current research shows that where
sprinklers and smoke vents are installed, the operation
of the ventilators in any one compartment does not
delay significantly the activation of sprinklers
(see Appendix 2 References 7 and 9).
The effect of the sprinklers on the temperature and
buoyancy of the smoke must be allowed for in the design
of the smoke extraction system, as the smoke may have
to pass through several rings of sprinklers before
being exhausted (see Appendix 2 Reference 10).
The designer should specify the allowances made for
sprinklers in the calculations.
9.2 High Racked Storage Warehouses
Unsprinklered high racked storage buildings have been
commonplace, but due to the fire experience in such
environments are now becoming less freguent. However,
if a design is required for such a building the fire
risk will be substantial, and experience has shown that
smoke ventilation has provided very limited additional
escape time, sufficient to save lives, but cannot be
depended upon to aid the fire brigade in fighting the
fire. Flashover in these circumstances is almost
certain to occur. The smoke ventilation system designer
should strenuously recommend the installation of a
sprinkler system.
The effect of sprinklers in a high bay warehouse has to
be considered in more detail than in other situations.
The type of sprinkler heads must be considered, eg.
Early suppression fast response ceiling mounted (ESFR)
Ordinary response ceiling mounted
Ordinary response in-rack
Fast response in-rack

The operating temperature of such heads must be
considered. Typical temperatures are.
68 degrees centigrade
93 degrees centigrade
141 degrees centigrade
The spacing of the sprinkler heads, eg.
Every storage level
Alternative storage levels
Roof level only
A combination of any of the above
Al1 of the above are alternatives and will affect the
design fire size. The designer should state within his
calculations what assumptions have been made.
9.2.1 Ceiling Level Sprinklers
Ordinary Response
With ordinary response ceiling mounted sprinklers
only, the water sprays will find it difficult to
penetrate into the middle of the racks.
Extinguishment will depend on water trickling
into the main seats of fire, with the consequence
that higher water flow rates will be needed.
Fire can be expected to "burrow" through shielded
areas of fuel until it reaches parts where
sprinklers are not yet wetting,the racks. At this
point the fire will flare up and set off more
sprinklers.
This behaviour was observed in some of LPC's
High-Piled palletted fires for the CEA. The
design fire size will be as hazard category 5
of Table 1 (see Section 4).
9.2.2 ESFR Sprinklers
Research carried out by others has indicated that
ESFR sprinklers appear to provide the best means
of control for a ceiling mounted system, for
buildings up to 12m in height. At or below this
height the design of the smoke control system can
be treated as for an in-rack sprinkler system.
There appears to be no research on the efficiency
of these systems above this height.

It therefore seems reasonable to treat the
sprinklers as being more effective than ordinaryresponse ceiling mounted systems, but less
effective than in-rack systems. In which instance
the design fire size adopted may be taken as a
mean value between the two.
The heat flux generated by the fire becomes less
important, as the effect of the sprinklers will
be to reduce the smoke layer temperature, and in
turn the smoky gas buoyancy. This must be
considered when natural ventilators are to be
installed, and the smoke layer temperature rise
above ambient should be limited to the minimum
sprinkler operating temperature minus ambient air
temperature. When mechanical ventilators are to
be used, it is recommended that a maximum of 50o
of this cooling should be considered. (This is
the average of the maximum calculated temperature
plus the sprinkler operating temperature).
The designer must state the degree of cooling
caused by the sprinklers which has been allowed
for in the calculations. Where there are extended
reservoir sizes (or no reservoir at all)
calculations of the additional heat lost from the
layer beyond the zone of sprinkler activity must
be stated.
9.2.3 In-Rack Sprinklers
Where there are sprinklers mounted in-rack, the
flame front rising up the rack may pass some
sprinkler heads before they operate. This effect
is particularly marked for face ignition. This
could lead to a fire which may be controlled, but
not extinguished, by the sprinklers when they do
operate, and the fire is likely to continue
burning.
In high racked fires the rising gases become
"channelled" by the flues formed by the racks.
The fire plume effectively has vertical sides
corresponding to the flues. The pattern of smoke
movement may be complicated by some smoke
spreading horizontally beneath shelves, but the
plume will rarely spread beyond a 2 bay width. It
does not spread at an angle of 15 degrees from
the vertical as is assumed for most axisymmetric
plumes in single storey buildings.

Heat may be generated through a significant
height of the rack, even when the fire is being
controlled by the sprinklers. This suggests a
useful analogy with the "Thomas" large-fire
plume, since its original derivation was for
those parts of a flame plume where combustion was
taking place in the gas phase, ie heat was being
generated throughout the height of plume. This
has the advantage that the large fire entrainment
model for a single storey building may be used.
The height of rise used should be taken as the
height from the lowest possible seat of fire (ie
usually the floor) to the base of the smoke
layer, and the perimeter of the fire must be that
part of the fire open to the air being entrained.
In practice this means that where the fire is in
the middle of a long rack, the only contribution
to the perimeter will come from the front 'face
of the rack (or if the fire is able to burn
through to both faces, from both front and back
faces of the rack). The lateral extent of fire
spread is less easy to assess, but should be
assumed to be the lateral separation between
sprinkler heads or two bay widths, whichever is
less - in most instances around 3m.
The fire 'size should therefore be taken as
either.
(a) One side of rack affected only
(b) Both sides of rack affected
The perimeter described by (b) should be used in
all cases. The only exceptions are when there is
a physical separation between the two faces of a
rack or when fast-response in-rack sprinklers are
installed to the precise specification and within
the limitations (experimental) laid down by the
FRS (Appendix 2 References 11-14) in these cases
perimeter (a) can be used.
During in-rack sprinkler tests at Cardington,
casual observation by FRS staff of the amount of
shimmer at a metre or two above the top of the
12m test rack suggested a gas temperature of
200C ± 50C when sprinklers were operating inrack. This estimate was unfortunately not
supported by any instrumentation at the time,
since it lay outside the design objectives of the
particular experiments. It is hoped that further
research will yield better data.

There is no indication that fire will progress
further up the racks, hence this value can be
taken as a limiting parameter, with the heat flux
being generated over a 12-14m height. Using the
previously described values of fire perimeter,
and the mass flow equation, the convective heat
flux generated can be approximated.
Where ceiling sprinklers are installed, either
complementary to, or in replacement of the
topmost in-rack sprinkler level, the final gas
layer temperature must be limited as described in
9.2.1 above, prior to further calculation of any
losses due to extended reservoir sizes.